As water beds have gained in popularity, many improvements have been made to overcome disadvantages that were present in the early primitive versions. A water bed consists of a rigid box-like, open top, frame which supports a generally flat envelope enclosing a volume of water. There are normally partitions or separations in the water containing envelope or mattress to prevent the water from shifting around excessively under the body of the user. One of the important features of a water bed is the means to heat the contents of the mattress to a temperature substantially above room temperature. Accordingly, it is desirable to provide an electric heater which has the capacity to heat the liquid contents of the mattress to a temperature of approximately 85.degree. Fahrenheit (29.4.degree. Centigrade).
The heater for the mattress is typically positioned on the upwardly facing surface of the mattress supporting frame with the mattress laying directly on top of the heater. It has been found that a normal water bed requires a heater having the capacity to deliver 300 or 400 watts in order to maintain the water at the desired 85.degree. F. temperature. There are some unusual requirements placed on the heater because of the environment in which it is located and the nature of the heat exchange and control problems encountered.
Although precise temperature control of the water in the mattress is not necessary, there are problems in controlling the heater which must heat the temperature of a large mass of water. The typical prior art water bed heater included a resistance heater similar to that which might be used in a heating pad but enclosed in a watertight envelope and controlled by a temperature probe located at a position spaced from the heating element and lying against the bottom of the mattress. Because of the thermal lag between the heater and the control, the heater would cycle over long time periods and had to be designed to operate on such long cycles without creating overheat problems.
Many types of heaters, if left on continuously, will have a tendency to create local overheating problems in the vicinity of the heater while the mass of the water in the mattress is still far below the desired temperature. This problem suggests that the heater must be somewhat distributed and not deliver the heat to too restricted a location or the material of the mattress would be damaged. There is no necessity that the heater be distributed entirely across the lower face of the mattress. The compromise as to the surface area of the heater engaged with the lower face of the mattress is largely a question of the materials used and the character of the heater.
There have been many serious problems involving the currently available resistance type water bed heaters having safety thermostats to guard against overheat conditions. The problem with such thermostats is that they cannot be made to respond to overheat conditions that may occur any place over the entire area of the heater. Accordingly, if the overheat occurs at a point away from the safety thermostat, damage may occur to the mattress or heater as a consequence of the overheat. To understand the nature of the problem, we need only look at the many possible causes of such overheat conditions.
If the user or installer of the water bed folds the heater or places some article of clothing between the heater and the mattress, an overheat will occur which may or may not be sensed by the safety thermostat before damage occurs. If the mattress is incompletely filled or used by someone who is very heavy, "bottoming out" takes place. This is a condition in which the user of the bed has his knee or posterior lying directly against the heater only separated by the top and bottom layers of the mattress envelope but with no water therebetween. This situation causes overheating and destruction of the mattress if the thermostat fails to respond.
It is also noted that it is important to have good heat exchange characteristics between the heater and the water contained in the mattress again to avoid local overheating in the area of the heater.
Another problem relating to the environment in which the water bed heater is placed relates to the risk of the mattress developing a leak and water being deposited in the area in which the heater is located. It is desirable, therefore, that any electrical heater located in such an environment be sealed and grounded to avoid the risk of delivering an electrical shock to the user of the bed.
As indicated above, some prior art water bed heaters have utilized conventional resistance type heating elements sealed in an envelope somewhat like the structure of a heating pad. There have also been attempts at fabricating water bed heaters of sheets of positive temperature coefficient (PTC) materials which have had patterns of electrodes deposited on one side thereof across the entire face of the sheet so that the sheet itself acts as a heater. Heaters of this general type are disclosed in the patents to Battiwalla, et al. No. 4,761,541 and No. 4,719,335 and to Grise No. 4,774,397. Other attempts have been made at depositing strips of PTC material between polyester sheets with spaced electrodes to supply current to the strips. Tests by applicant of these various types of water bed heaters made using sheets or layers of PTC material have indicated many shortcomings in these heaters. At the present time there are no practical or commercially successful water bed heaters on the market using PTC. Some tended to have low breakdown temperatures, and others exhibited negative temperature coefficient characteristics when heated for prolonged periods of time. The studies made of the commercially available water bed heaters indicated that there existed a need for a reliable and safe water bed heater which would operate in a foolproof manner to maintain the water in the mattress at approximately 85.degree. F. while eliminating any risks of shock to the user in the event of water leakage from the mattress.
Other prior art patents of interest are the patents to Leary, et al. No. 4,425,497 and No. 4,547,659 which disclose PTC heaters sandwiched between aluminum sheets to increase power output. Also of interest is the patent to Waltz No. 4,314,231 which discloses a PTC heater with mesh electrodes enclosed in an envelope of polymeric insulating layers.
Of increasing concern in recent years is the possibility that the electromagnetic fields associated with current carrying wires may in some way be injurious to the health of a human exposed to such fields. This concern is somewhat greater with respect to appliances or products where the exposure is greater and continues over a longer period of time as with electric heaters associated with water beds. There have been no prior art water bed heaters which address the problem of electromagnetic radiation or have included any means to reduce such radiation.